How Snake Venom Evolved: Genes Reveal It Wasn't Always Toxic

Is an animal venomous or isn't it? That question is more complicated than you might think. Gene mapping has changed the way scientists define an animal as venomous; now, though, a new study challenges the practice of gene mapping while also developing a new model of how venom first evolved.

When it comes to defining whether or not an animal is venomous, scientists traditionally examine the oral glands. If these glands show expression of some of the 20 gene families associated with venom toxins, then that species is venomous. Yet this may not be the best way to label a venomous animal.

In order to learn a bit more about venomous species, the researchers compared groups of related genes in tissue from different parts of the Burmese python. They found similar levels of so-called toxic gene families in python oral glands and in tissue from the python brain, liver, stomach and several other organs. This, in particular, shows much about the function of venom genes before they evolved into venoms.

"Research of venom is widespread because of its obvious importance to treating and understanding snakebite, as well as the potential of venoms to be used as drugs but, up until now, everything was focused in the venom gland, where venom is produced before it is injected," said Todd Castoe, one of the researchers, in a news release. "There was no examination of what's happening in other parts of the snake's body. This is the first study to have used the genome to look at the rest of that picture."

The findings suggest that highly toxic venom proteins were evolutionarily "born" from non-toxic genes, which have other ordinary jobs around the body, such as the regulation of cellular functions or digestion of food.

More specifically, the researchers created a model with three steps for venom evolution. First, these potentially venomous genes end up in the oral gland by default, because they were expressed in low but consistent ways throughout the body. Then, natural selection caused these genes to be expressed in higher levels in tissues in the mouth than in other parts of the body. Finally, as the venom became more toxic, the expression of these genes in other organs decreased to limit potentially harmful effects.

The findings reveal a bit more about the evolution of venom and how it first came to be in snakes. Currently, the scientists plan to see if this model also works in snakes that are highly venomous.

The findings are published in the journal Molecular Biology and Evolution.

For more great science stories and general news, please visit our sister site, Headlines and Global News (HNGN).